Not applicable.
The present invention relates to a procedure for generating and maintaining turbulence in stock suspension flow conducted through a turbulence generator into the slice duct of the headbox and therefrom through the slice opening to the web former, in which procedure the stock suspension flow is with the aid of turbulence pipes divided into a number of superimposed layers, whereafter the effect of the elements creating and maintaining turbulence is directed thereupon.
The invention also relates to a turbulence generator of the headbox of a paper machine, comprising a number of overlapping turbulence pipes being arranged in rows and extending across the entire width of the headbox, through which pipes the stock suspension flow to be conducted from the headbox to the web former is arranged to flow and which turbulence pipes are provided with stepwise expansion of the flow cross-section area between the inlet and outlet of the pipe, and to which turbulence generator a plurality of headbox dividers or lamellae can moreover be connected, starting from between the pipe rows and extending to the slice duct of the headbox.
It is of vital importance, considering the quality of the paper/board being manufactured, to understand what kind of turbulence spectrum of stock suspension flow prevails in the slice duct of the headbox and in the subsequent web former. The turbulence generated with the aid of the turbulence generator in the stock suspension flow will decrease quite rapidly unless turbulence energy is continuously added in the flow. The formation of paper or board is best enhanced by small-scale vortices which efficiently disintergate fibre bundles. Large-scale vortices may even be detrimental considering the formation of paper. Owing to the properties of the turbulence, the small-scale vortices are first to reduce in the flow, whereby, for instance, the surface layer of the web on the Fourdrinier wire and the middle layer of the web on a gap former tend to be more flocculated than the other layers due to decreasing turbulence. A generally employed manner to increase turbulence energy in the flow by using the draw between the slice jet and the wire does not act in the area being dewatered last. In order to have more turbulence in said area, the draw is to be great. Hereby, the formation of the area dewatered first is easily deteriorated to the extent that the formation of the entire product can no longer be improved. A similar progress may also occur when endeavours are made in the web former to introduce turbulence energy into a stock suspension layer not yet dewatered, e.g. by means of loading lists through a layer already dewatered.
In a majority of the state-of-art turbulence generators, all turbulence pipes are mutually identical because the aim is to achieve homogeneous turbulence in different parts of the stock flow. Such turbulence generators make no difference between the bottom, surface and middle layers of the web. In web formation, said layers become, however, dewatered at different times. On the Fourdrinier wire, the surface layer is dewatered last and in the gap former the layer to be dewatered last is the middle layer.
In patent specification U.S. Pat. No. 5,124,002, a turbulence generator is disclosed in which the flow cross-section areas of the turbulence pipes in superimposed layers differ in size and shape, and advantageously, the mutual spaces between the pipes are also different. In this manner, a different microturbulence level can be generated in different layers of the stock suspension flow discharging from the turbulence generator into the slice duct, and such paper can be manufactured which is provided with different fibre orientations in superimposed layers. The flow cross-section area of each turbulence pipe remains the same from the first part of the pipe to the end thereof.
Such turbulence generators are also known in the art in which the flow cross-section area of the turbulence pipes is step-wise expanded at least at one spot between the inlet and the outlet of the pipe. In the turbulence generators known in the art, the expansion spots of the pipe are at equal distance from the outlet of the pipe in all pipes. One such prior art design is disclosed in U.S. Pat. No. 5,183,537.
The objective of the present invention is to develop a new procedure for generating and maintaining turbulence and a new kind of turbulence generator, with the aid of which a different turbulence can be generated in different layers of stock suspension flow flowing out of the headbox.
One more aim of the invention is to achieve an application in which the turbulence of the stock suspension layer dewatered last in the former after the headbox can be maintained closer to the optimal level during the formation than with currently used turbulence generators. Thus, the aim is a stock suspension flow in which the turbulence is xe2x80x9cfreshestxe2x80x9d, and consequently, most lasting in the layers of the flow which stay xe2x80x9crunningxe2x80x9d longest. When the impact of the factors generating turbulence in the flow ceases, the turbulence begins to slow down rapidly. The turbulence is the fresher the shorter length the flow has propagated after the generation of turbulence.
To achieve said objectives and those to be disclosed below, the procedure of the invention is characterized in that turbulence is generated in different layers of the flow in different phases of the flow by arranging the elements generating and maintaining turbulence at different distances from the slice opening of the headbox, so that a different turbulence prevails in different layers of the stock suspension flow.
Respectively, the turbulence generator of the invention is characterized in that the distance of the expansion spot of the turbulence pipes in superimposed pipe rows from the slice opening of the headbox and/or the distance of the tips of the trailing elements in association with the pipe rows from the slice opening of the headbox is different so that at the slice opening, the turbulence is different in different layers of the stock suspension flow.
In an advantageous embodiment of the invention, the expansion spots of individual turbulence pipes of a turbulence generator are so stepped that in the superimposed turbulence pipe rows, the expansion of the flow cross-section area is carried out at a different distance from the slice opening of the headbox. The later the phase is in which the cross-section area of a turbulence pipe expands, the fresher is the turbulence as the stock suspension flow discharges from the slice opening of the headbox onto the forming wire or into the gap between the forming wires. The expansion spots of the turbulence pipes acting on the layer of the stock suspension flow to be dewatered last are arranged to be last in the flow direction, that is, closest to the slice opening.
In addition to stepping the expansion spots, or instead of it, a different turbulence can be generated in different layers of the stock suspension flow by providing, after the turbulence pipes, trailing elements extending to the slice duct, which in superimposed flow layers extend to a different distance from the slice opening of the headbox. The trailing elements can be fixed in length or their lengths can be adjustable, as in U.S. Pat. No. 4,133,713. Alternatively, the fixing point of a trailing element in the longitudinal direction to the headbox can be adjustable, as in FI patent specification No. 88317. The purpose of the trailing elements is to keep different layers of the stock suspension flow separated as long as possible after a different turbulence has first been generated in the layers, for example, by stepping the expansion parts or by employing turbulence pipes differing in the flow cross-section area. The trailing elements maintain and strengthen the difference of turbulences prevailing between different layers. Alternatively, all trailing elements can be mutually of equal length, whereby various levels of turbulence prevailing in different layers can be achieved solely with the aid of structural differences of turbulence pipes.